The invention is concerned with heating apparatus, particularly microwave heating apparatus.
This type of apparatus is used extensively for the heating, thawing or cooking of foods including pre-packaged food portions. In the context of the heating of pre-packaged food portions in large quantities it is essential to ensure that the heating is uniform not only between individual packaged portions but also within each portion.
It is well known that microwave ovens of the multimode type do not achieve satisfaction uniformity of heating for the following reasons:
1. Multiple reflections from the walls of the oven create standing waves of voltage stress which give rise to a pattern of hot spots.
2. As the microwave field enters the product it is attenuated by the power absorption and consequently may be reduced to a low intensity by the time it has penetrated to the central interior of the product.
3. The high dielectric constant of the product causes total internal reflection resulting in energy being "trapped" inside the product and causing standing waves within it.
Moreover the high dielectric constant of most food products creates refraction causing preferential heating along the edges and particularly at corners of three dimensional projections.
Where continuous flow of heating of discrete objects is required, it has been found that a multimode oven cannot provide the required degree of uniformity usually required in an industrial process, e.g. in the pasteurisation of food products it is essential that the temperature shall reach at least a minimum value to achieve the required low bacterial count but the temperature should not reach a value at which the product is cooked or in an extreme case partially dried. Other products may partially melt or decompose to create off-flavours if the temperature exceeds a modest maximum. This means that the energy input must be uniform with a range +/-2% to +/-10% from point to point within the product, depending upon the precise requirements.